Laminated electrical circuit substrates which include a conductive layer supported on a dielectric fluoropolymer matrix composite layer are known. Driven by the continuing trend toward increasing circuit density, very thin films, e.g. less than about 1.0 mil, of highly filled fluoropolymeric matrix composite substrate materials having a substantially uniform microstructure have become desirable as allowing further reduction in the size of electronic circuits. It is technically and economically difficult to make such materials by known methods.
Fluoropolymer and particulate filled fluoropolymer matrix composite films are made by known papermaking, skiving, casting, melt extrusion and paste extrusion and calendering processes.
Films produced by paper making processes required fiber reinforcement and are limited to thicknesses greater than about 2 mil.
It is very difficult to produce thin high quality highly filled fluoropolymer matrix films by skiving due to abrasion of the skiving blade by the filler particles and tearing of the film associated with the resistance of the filler particles to the skiving blade.
The filler loading of films made by known casting processes is limited to less than about 15 volume percent.
The high melt viscosity of neat fluoropolymers complicates the production of fluoropolymer films by melt extrusion. Polyvinylidene fluoride (PVF.sub.2) and polychlorotrifluoroethylene (PCTFE) are melt extrudable only within a narrow processing window. Polyvinylfluoride (PVF) film cannot be produced by melt extrusion due to thermal instability. Polytetrafluoroethylene (PTFE) cannot be melt extruded due to its extraordinarily high melt viscosity. Fluorocopolymers are known which provide lower melting temperature and lower melt viscosity at extrusion temperatures, e.g. copolymers of tetrafluoroethylene with hexafluoropropylene (FEP) or with ethylene, copolymers of CTFE with vinylidene fluoride or hexafluoropropylene.
The introduction of fillers further complicates the melt extrusion of fluoropolymers. In the presence of certain fillers, especially at high filler loading level, the melt processability of the melt extrudable fluoropolymers is rapidly degraded due to the increase in melt viscosity associated with the presence of the filler or with filler-catalyzed thermal degradation of the polymer matrix.
A method of making highly filled PTFE composite materials which exhibit excellent physical and electrical properties by paste extrusion and calendering is set forth in coassigned U.S. Pat. No. 4,849,284 to D.J. Arthur, J.C. Mosko, C.S. Jackson and G.R. Traut, entitled "ELECTRICAL SUBSTRATE MATERIAL", the disclosure of which is incorporated herein by reference. However, it is technically difficult and economically extremely difficult to produce thin, i.e. less than 2 mils, highly filled, i.e. greater than about 40%, fluoropolymer matrix composite films by the paste extrusion and calendering process.
What is needed in the art is a method which overcomes the above noted deficiencies of known processing methods.